ARM: dts: imx6dl-yapp4: Use aliases to set custom MMC device indexes

Until commit fa2d0aa9 ("mmc: core: Allow setting slot index via
device tree alias") was introduced, our usdhc3 and usdhc4 devices
were enumerated as mmc0 and mmc1. The mmc1 device is used to boot/update
the board and its name must be fixed.

With the referenced commit, aliases from imx6qdl.dtsi took effect.
Override the aliases to get back the original device indexes.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>